1. Field of the Invention
This invention relates to automatic spray systems in which a number of spray heads traverse a medium to produce a color image in response to a coded input. In a particular system, an image to be copied is scanned optically and the signals from the scanner control the characteristics of each of the spray jets to accurately reproduce the image. Such spray systems are used in preparing outdoor advertising displays, for the reproduction of art work and for other applications. More particularly, the invention relates to an automatic spray system in which a test image formed under the control of a computer for each basic cole, is scanned by a densitometer and the information from the densitometer utilized for preparing a set of look-up tables that relate the spray characteristics of each jet spray head to reproduce the appropriate color density.
2. Description of Related Art
Many different kinds of automatic spray systems have been devised. U.S. Pat. No. 1,709,926 describes a system in which three separate jets, each spraying one of the primary colors, are individually controlled by electrically-operated valves to produce a blended color image. U.S. Pat. No. 3,553,371 describes a multi-jet spray system in which the flow of ink from each of the jets is controlled either by controlling the air pressure that produces the spray or by direct control of the ink supply. The medium to be sprayed is carried by a rotating drum and the jets are moved laterally across the face of the drum to produce a spiral pattern. U.S. Pat. No. 4,914,522 describes a multi-jet spray system in which the density of each color sprayed on the medium is controlled by pulse-width modulation of an air stream that atomizes the ink. These and many other systems for controlled spray painting of mediums have been devised. Throughout these developments, the accurate reproduction of colored images has remained a recurrent problem. In systems in which two sets of jets produce an interleaved scanning pattern, it is even more important to maintain precise control over the color density produced by each of the jets.
The spray characteristics of the individual jets are quite different. Slight variations in the dimensions or shape of jet components influence the amount and format of the delivered spray. Moreover, the characteristics of a single jet may change during a period of extended use, for example, from the drying of ink on or adjacent to the jet. Any such variations cause a variation in the rate at which the ink is delivered to the medium and a change in the color density of the pattern. To reproduce a colored image with accuracy, it is necessary that each jet be individually controlled in such manner as to produce the prescribed color density. In systems in which two sets of heads scan alternate lines of the image, any variation between the operating characteristics of the two sets of heads will produce lining on the image. Thus, even if the adjustment is such as to provide acceptable color reproduction, the image may be impaired by the presence of lining in the image caused by the slightest variations in the spray characteristics of the two sets of heads. In an automatic spray system in which color density is controlled by pulse width modulation, it is necessary to have an absolute correlation between the pulse width and the resulting color density.